The field of the invention is that of surface acoustic wave transducers operating at very high frequencies, typically at-frequencies of the order of several gigahertz.
Conventionally, transducers are typically fabricated by using structures of two, four or eight electrodes per wavelength xcex, where xcex corresponds to the central operating frequency of the transducer, depending on the intended applications (transducer with or without reflection). In all these types of transducers, the ratio of the area of the metallized surfaces of the substrate to the area of the free surfaces generally used is typically between 0.25 and 0.75.
A new class of transducers has now come into being. These are the so-called small-gap transducers, in which the free surface is very small so as to obtain the smallest possible distance between two consecutive electrodes as illustrated in FIG. 1. The advantages of this type of transducer lie in the fact that it is possible to obtain electrode widths which are as large as possible, per period, therefore with strongly decreasing reflection phenomena between electrodes. The drawback of these structures lies in the technological difficulties. By way of example, for a transducer operating at 16 GHz, electrodes of width xcex/2, i.e. 1.5 xcexcm, have to be separated by a distance of about a few hundred angstroms, which requires an exacting technology. Various technologies have been proposed and especially that proposed by J. H. Hines and D. C. Malocha, IEEE MTT-S International Microwave Symposium Digest (Cat. No. 97CH3607), (1997), Denver, Colo., USA, Jun. 8-13, 1993). According to this concept, the conventional production of electrodes is carried out according to the metallization/free surface ratio of about 50/50, as illustrated in FIG. 2a. The whole of this first series of electrodes is covered with a layer of dielectric material (FIG. 2b). A second series of electrodes is then produced as illustrated in FIG. 2c. It is thus possible to obtain electrodes separated by a very small distance corresponding to the thickness of the layer of dielectric material, however the second series of electrodes is not in direct dielectric contact with the piezoelectric substrate, thus generating a coupling of lower efficiency.
Furthermore, this technology inherently leads to structures without possible reflection and therefore, necessarily, to bidirectional transducers.
In this context, the invention proposes a novel type of transducer of the small-gap transducer type, obtainable by a simple technology and leading to transducers which do not have a coupling problem. According to the invention, the gap between consecutive electrodes is obtained in a direction perpendicular to the plane of the substrate and by virtue of the presence of an etched grating.
More specifically, the subject of the invention is a surface acoustic wave transducer operating at frequencies of the order of a few gigahertz and comprising a substrate on which at least two interdigitated electrode arrays are deposited and connected to different polarities, so as to create acoustic transduction cells, which are defined by at least two consecutive electrodes of different polarities, characterized in that it comprises:
a grating of etched grooves separated by mesas, in the substrate;
all of the mesas and of the etched grooves being entirely covered with a metallization layer making up the electrodes;
the depth of the etched grooves being greater than the thickness of the metallization layer so as to create a gap, of the order of a few hundred angstroms in a direction perpendicular to the plane of the substrate, between two consecutive electrodes.
This type of transducer has the benefit of being able to be produced directly. This is because, after having firstly made an etched grating with the desired periodicity, it is enough to subsequently proceed to the metallization of the whole of the surface of the etched grating.
Note that, in order to minimize the sensitivity to the production, it may be beneficial to make the mesas wider than the etched grooves.
Advantageously, the small-gap transducer according to the invention may have a favored acoustic wave propagation direction.
Note that in the known art it has already been proposed to produce transducer architectures using mesas and etched grooves (WO 82/01629) but aiming for other application fields/transducers operating at megahertz frequencies, with much larger gaps and with metallizations that are not continuous between mesas and etched grooves.
Thus, according to a first variant of the invention, the distance between two consecutive electrodes is equal to xcex/4, and is also equal to the distance between a mesa and a consecutive etched groove. The sidewalls of the etched groove create reflections which make it possible for the transmitted waves to be returned in phase with the reflected waves in a favored direction and in the phase opposition in the opposite direction so as to obtain a unidirectional transducer, also called a SPUDT (Single-Phase Unidirectional Transducer). In this type of transducer, it has moreover been demonstrated that it could be beneficial to produce resonant cavities inside said transducer, this is described in particular in the published patent application Ser. No. 2,702,899. In order to create a resonant cavity, it is necessary to locally modify the directivity of the surface acoustic waves. To do this, the phase is conventionally varied by shifting one electrode locally, something which is not straightforward from a technological point of view.
According to the invention, it becomes very simple to change the directivity, by locally making an etched groove of larger width, typically twice as large.
According to a second variant of the invention, the small-gap transducer is a bidirectional transducer having a period of xcex/2. Generally, this type of transducer has a very good coupling coefficient and a critical dimension in xcex/2 technology, and therefore is easier to produce than the transducers having a configuration of four electrodes per xcex.
According to a third variant of the invention, the small-gap transducers comprises etched grooves and mesas of xcex/3 period. This type of transducer has the advantage of strongly minimizing the marked reflections from electrodes, the objective sought after in certain applications.